|
(11) | EP 0 720 197 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||
| (54) | Directly heated cathode structure |
| (57) A directly heated cathode structure includes a porous pellet (500) impregnated with
electron radiating material, a cup-shaped container (510) for holding the porous pellet
(500), a metal member (520) being welded at the base of the container (510), and a
filament (600) arranged between the container (510) and the metal member (520), thereby
restricting thermion emission through the base and sides of the pellet (500) and extending
the life of the cathode structure. |
[0001] The present invention relates to a directly heated cathode structure for a cathode-ray tube (CRT), and more particularly, to a directly heated dispenser cathode structure for use in a color CRT electron gun.
[0002] A cathode absorbs heat energy and emits thermions. Generally, cathodes may be divided into directly heated and indirectly heated types, according to the heating manner of the emitting source material. In a directly heated cathode, the filament and emitting source are in direct contact with each other, while in an indirectly heated cathode they are separated.
[0003] The directly heated cathode is most often used in an electron gun of a small CRT such as is used in a viewfinder of a video camera, and is directly fixed to a filament and provided with a base metal whose surface is coated with electron-radiating material or a pellet into which cathode material is impregnated may be used for an electron gun of a large CRT for a TV or a computer monitor. A porous pellet structure fixed directly to the filament has been developed by the present applicant (US patent application serial No. 08/120,502), as shown in FIG. 1. Here, a single filament 102 penetrates a porous pellet 101 in which electron-radiating material is impregnated. Alternatively, a pair of such filaments are directly welded to the sides of the porous pellet.
[0004] The present applicant has also filed a patent application (US patent application serial No. 08/429,529) describing a cathode structure in which the supporting structure of a pellet is reinforced by the filaments themselves. The filaments are in this case directly welded to (or penetrate at) at least three points on the outer sides of a porous pellet in which electron-radiating material is impregnated.
[0005] The above-mentioned directly heated cathode structures need only a very short interval after current is applied before starting thermion emission and exhibit high-density thermion emission, since the porous pellet is directly heated by the filament current with the filament being in contact with the body thereof. However, there is a loss in the thermion emitting material since thermion emission is made through the entire surface of the pellet (including the sides thereof), and the thermion radiating material evaporated from the pellet to the filament can embrittle the filament. Also, the process of attaching the filament to the pellet (either by welding thereto or by passing it through the pellet) is difficult to achieve in practice, resulting in lower productivity.
[0006] The present applicant has furthermore developed a directly heated cathode having an improved structure, as shown in FIG. 2. Here, a filament 210 is fixed to a metal member 220 which is arranged under a pellet 200 where electron radiating material is impregnated. Thus, since metal member 220 covers the base of pellet 200, thermion emission through the base of pellet 200 is effectively blocked.
[0007] However, a small portion of the thermions escape through minute gaps which exist between pellet 200 and metal member 220. Moreover, since the pellet sides also constitute thermion emission surface area, continuous and uniform thermion emission cannot be achieved. Further, the life of pellet 200 is shortened due to the rapid consumption of the electron radiating material, and, as in the case of the aforementioned structure, the electron radiating material evaporated from the sides of pellet 200 can embrittle the filament.
[0008] To solve the above problems, it is an object of the present invention to provide a directly heated cathode structure by which emission through the base and sides of a pellet is restricted.
[0009] It is another object of the present invention to provide a directly heated cathode structure of a high quality which imparts improved stability and enables greater productivity.
[0010] Accordingly, there is provided a directly heated cathode structure comprising: a porous pellet where electron radiating material is impregnated; a cup-shaped container for holding the porous pellet; a metal member being welded at the base of the container; and a filament arranged between the container and the metal member.
[0011] Specific embodiments of the present invention are described in detail below, with reference to the attached drawings, in which:
FIG. 1 is a perspective view illustrating a conventional directly heated cathode structure;
FIG. 2 is a section illustrating another conventional directly heated cathode structure;
FIG. 3 is a schematic perspective view illustrating a directly heated cathode structure according to the present invention;
FIG. 4 is an exploded perspective view illustrating the directly heated cathode structure of FIG. 3; and
FIG. 5 is a section view illustrating the directly heated cathode structure of FIG. 3.
[0012] Referring to FIGS. 3-5, electron radiating material is impregnated into a porous pellet 500 of metal having a high melting point. Porous pellet 500 is inserted into a cup-shaped container 510 for protecting pellet 500 by enclosing the sides and base thereof. A filament 600 is provided under container 510. Under filament 600, a metal member 520 is provided for fixing the filament to the base of container 510. Both filament 600 and metal member 520 are fixed to the base of container 510 by welding.
[0013] Here, the porous pellet 500 is made of tungsten (W), ruthenium (Ru), molybdenum (Mo), nickel (Ni) and/or tantalum (Ta), and the material used for container 510 and metal member 520 includes molybdenum (Mo), tungsten (W) and/or tantalum (Ta).
[0014] In a specific embodiment of the present invention, container 510 containing pellet 500 has an inner diameter of 0.50-2.00mm, and the appropriate thickness of container 510 is 0.02-0.50mm. Container 510 can be a cylindrical column and may also be rectangular or polygonal in section. For the material of filament 600, it is preferred to use a Re-alloy, of which the main constituent is tungsten or molybdenum. It is also preferred that the diameter of the filament is 0.02-0.50mm. Metal member 520 has a shape corresponding to that of the base of container 510, preferably with a diameter and thickness matching those of the container.
[0015] For the welding of container 510 and metal member 520, resistance welding, laser welding, arc welding or plasma welding can be used. It is preferred that two or more filaments are arranged cross-wise or radially, for more efficient pellet heating.
[0016] The directly heated cathode structure according to embodiments of the present invention has the following merits.
[0017] Firstly, since the pellet where electron radiating material is impregnated is held and protected in the container, oxidation of the electron radiating material due to the welding heat generated during the welding of the container and metal member, can be prevented.
[0018] Secondly, since the filament is welded to the container containing the pellet, the binding strength between the pellet and the filament can be improved.
[0019] Thirdly, since the pellet is held in the container only the top side of which being exposed, the vaporization of the thermion emission material is minimized, so that a shortening of the cathode's life can be prevented.
1. A directly heated cathode structure comprising:
a porous pellet impregnated with electron radiating material;
a cup-shaped container for holding said porous pellet;
a metal member welded at the base of said container; and
a filament arranged between said container and said metal member.
a porous pellet impregnated with electron radiating material;
a cup-shaped container for holding said porous pellet;
a metal member welded at the base of said container; and
a filament arranged between said container and said metal member.
2. A directly heated cathode structure as claimed in claim 1, wherein said filament is
constituted by plurality of filament members in a radial arrangement.
3. A directly heated cathode structure as claimed in claim 1 or claim 2, wherein said
pellet is manufactured using at least one metal selected from the group consisting
of tungsten (W), ruthenium (Ru), molybdenum (Mo), nickel (Ni) and tantalum (Ta).
4. A directly heated cathode structure as claimed in any preceding claim, wherein the
main constituent of said filament is tungsten (W) and a subsidiary constituent is
rhenium (Re).
5. A directly heated cathode structure as claimed in any preceding claim, wherein the
diameter of said filament is 0.02-0.50mm.
6. A directly heated cathode structure as claimed in any preceding claim, wherein said
container is manufactured using at least one metal selected from the group consisting
of molybdenum (Mo), tungsten (W) and tantalum (Ta).
7. A directly heated cathode structure as claimed in claim 6, wherein the thickness of
said container is 0.02-0.50mm.
8. A directly heated cathode structure as claimed in any preceding claim, wherein said
metal member is manufactured using at least one metal selected from the group consisting
of molybdenum (Mo), tungsten (W) and tantalum (Ta).
9. A directly heated cathode structure as claimed in claim 8, wherein the diameter of
said metal member is 0.50-2.00mm and its thickness is 0.02-5.00mm.
10. A directly heated cathode structure as claimed in any preceding claim, wherein the
shape of said pellet is cylindrical.
11. A directly heated cathode structure as claimed in any preceding claim, wherein the
shape of said pellet forms a polygonal column.